Observer Pattern

The observer pattern gives you another way to avoid tight coupling between components. This pattern is simple: One object makes itself observable by adding a method that allows another object, the observer, to register itself. When the observable object changes, it sends a message to the registered observers. What those observers do with that information isn’t relevant or important to the observable object. The result is a way for objects to talk with each other without necessarily understanding why.

It’s also known as the Subscribe/Publish Pattern

The Observer pattern is perhaps most often encountered in traditional graphical user interface desktop applications. It is an excellent way of ensuring that disparate display components reflect changes at a system’s core. As we shall see, though, it remains a powerful technique in a Web-oriented environment.